Methods for improving uplink coverage, throughput and transmission latency are currently being investigated in third generation partnership project (3 GPP) in the context of the Release 6 (R6) universal mobile telecommunications system (UMTS) study item “FDD uplink enhancements”.
It is widely anticipated that in order to achieve these goals, Node-B (base station) will take over the responsibility of scheduling and assigning uplink resources (physical channels) to users. The principle is that Node-B can make more efficient decisions and manage uplink radio resources on a short-term basis better than the Radio Network Controller (RNC), even if the RNC retains coarse overall control. A similar approach has already been adopted in the downlink for Release 5 (R5) high speed downlink packet access (HSDPA) in both UMTS FDD and TDD modes.
It is also envisioned there could be several independent uplink transmissions processed between a wireless transmit/receive unit (WTRU) and a universal terrestrial radio access network (UTRAN) within a common time interval. One example of this would be medium access control (MAC) layer hybrid automatic repeat request (HARQ) or simply MAC layer automatic repeat request (ARQ) operation where each individual transmission may require a different number of retransmissions to be successfully received by UTRAN. To limit the impact on system architecture, it is expected that protocol layers above the MAC should not be affected by introduction of the EU-DCH. One requirement that is introduced by this is the in-sequence data delivery to the radio link control (RLC) protocol layer. Therefore, similar to HSDPA operation in the downlink, a UTRAN re-ordering function is needed to organize the received data blocks according to the sequence generated by the WTRU RLC entity.
In a conventional wireless communication system based on downlink HSDPA operation, new MAC entities for the EU-DCH in the WTRU and Node B are required. The Node B MAC entity would be responsible for scheduling and assignment of physical resources, and the re-ordering function would be incorporated in the system for in-sequence delivery to the RNC.
FIG. 1 is a signal flow diagram depicting the operation of a conventional wireless communication system 100 in which out-of-sequence delivery to an RLC entity in the serving-RNC (S-RNC) and RLC recovery occur on the WTRU side during an EU-DCH inter-Node-B serving cell change. The wireless communication system 100 includes a WTRU 105, a target Node-B 110, a source Node-B 115 and an S-RNC 120.
Still referring to FIG. 1, when the S-RNC 120 realizes a need for an EU-DCH inter-Node-B serving cell change (step 125), the S-RNC sends an Iub request message 130 to the target Node-B 110. The target Node-B 110 is informed of the EU-DCH inter-Node-B serving cell change and a MAC entity is set up (step 135). The target Node-B sends an Iub response message 140 to the S-RNC 120 which, in turn, sends a radio resource control (RRC) request message 145 to the WTRU 105. The EU-DCH inter-Node-B serving cell change is realized in the WTRU 105, whereby HARQ processes and transmission sequence numbers (TSNs) are reset (step 150). The WTRU 105 then sends an RRC complete message 155 to the S-RNC 120 which, in turn, sends an Iub request message 160 to the source Node-B 115. The source Node-B 115 is informed of the EU-DCH inter-Node-B serving cell change and the re-ordering buffer is flushed (step 165). The source Node-B then sends an Iub response message 170 to the S-RNC and an out-of-sequence delivery message 175 to the RLC in the S-RNC 120. An RLC status report message 180 is then sent from the S-RNC 120 to the WTRU 105 to initiate an RLC recovery process 185.
Since the EU-DCH inter-Node-B serving cell change results in switching from one Node-B to another, and the re-ordering queue status is only known to the source Node-B, it is necessary to reset the HARQ processes and TSNs in the WTRU 105, and flush the re-ordering queues in the source Node-B 115. This results in out-of-sequence delivery to higher layers and significant delay in recovering data lost in the WTRU 105.
An example of out-of-sequence delivery to RLC and RLC recoveries caused EU-DCH inter-Node-B serving cell change in the conventional wireless communication system 200 is shown in FIG. 2. The wireless communication system 200 includes a WTRU 205, a target Node-B 210, a source Node-B 215 and an S-RNC 220.
Before the EU serving cell is changed, protocol data units (PDUs) with sequence numbers (SNs) 1-5 are sent from a data buffer 225, located in the WTRU 205, to the source Node-B 215. However, in the example shown in FIG. 2, only the PDU with SNs 1, 3 and 4 are received correctly by the source Node-B 215 and stored in a re-ordering buffer 230 in the source Node-B 215. Thus, in this example, the PDUs with SNs 2 and 5 are missing.
Still referring to FIG. 2, after the EU serving cell is changed, the HARQ processes and SNs in the WTRU 205 are reset (step 235), and the re-ordering buffer 230 in the source Node-B 215 is flushed (step 240). In step 245, an out-of-sequence delivery, (i.e., PDUs 1, 3, 4), to the RLC in the S-RNC 220 occurs. The RLC in the S-RNC 220 then generates a first RLC status report message 250 requesting PDUs associated with the old SN 2. The terminology “old” refers to the fact that the PDU with SN 2 is missing in the source Node-B 215 before handover. In response to receiving the message 250, the WTRU 205 transmits the PDUs, associated with the old SN 2, with a new SN 1 to a re-ordering buffer 285 in the target Node-B 210 (step 285). Additionally, the WTRU 205 transmits the PDUs, associated with the old SN 6, with a new SN 2 to the re-ordering buffer 285 in the target Node-B 210 (step 258). The new SN 1 and SN 2 are then forwarded to the RLC in the S-RNC 220 (respective steps 285 and 285). In step 285, an out-of-sequence delivery to the RLC in the S-RNC 220 occurs again. The RLC in the S-RNC 220 then generates a second RLC status report message 285 requesting PDUs associated with the old SN 5. In response to receiving the message 285, the WTRU 205 transmits the PDUs, associated with the old SN 5, with a new SN 3 to a re-ordering buffer 285 in the target Node-B 210 (step 290). The new SN 3 is then forwarded to the RLC in the S-RNC 220 (step 295).
The conventional systems 100, 200, shown in FIGS. 1 and 2, respectively, experience significant delays due to flushing a re-ordering buffer and recovering PDUs from the WTRUs 105, 205. It is desired to reduce such delays.